When your metabolism is in “rest” mode, you burn mostly glucose, the simple sugar that’s found in your blood. Your fat cells remain untouched, and you don’t lose any weight. But when you engage in sustained, strenuous (i.e., aerobic) exercise, your system shifts gears and you start metabolizing body fat. This typically occurs about 15 or 20 minutes into your workout, when you experience the familiar phenomenon known as “hitting your stride.” Suddenly, things seem easier. That’s because fat is a more efficient fuel–it provides 18 times more energy than glucose.

Your metabolic rate fluctuates during the day, peaking out around midday. Studies indicate that a load of calories taken at night will result in more weight gain than the same amount taken in the morning. So if you must make a pig of yourself, do it before noon. Morning is also the best time to exercise if you want to lose weight. One study indicated that two-thirds of the calories you burn up in the morning come from fat, whereas less than half come from fat in the afternoon. The act of eating itself kicks up your metabolic rate 5 to 30 percent. This has led some nutritionists to suggest that you ought to eat lots of little meals during the day rather than one or two big ones.

Thank you, Cecil at The Straight Dope